The prosthetic member of the thumb joint prosthesis includes a joint socket member (6) having a threaded portion (4) and a mounting pin (9) that can be inserted into the hole (8) of the threaded portion for mounting the joint socket member. ). The threaded portion (4) of the prosthetic member (2) comprises a first outer thread (17) for attaching the prosthetic member to the rigid outer portion (T1) of the great rhomboid bone (T) by screwing and tightening. The first fixing part (15) has a diameter smaller than the diameter of the first fixing part, and is configured so that the prosthetic member can be attached to the soft inner part (T2) of the rhomboid bone by screwing and tightening. And a second fixing part (18) provided with a second outer thread (19). The first fixing portion (15) connects the threads (17, 19) of the fixing portions (15, 18) to each other and converges in a substantially arc shape toward the second fixing portion (18) ( 20). This achieves optimal fixation of the prosthetic member (2) in the great rhomboid bone (T). [Selection] Figure 5親指関節用人工関節の補綴部材は、ねじ状部（４）と、関節ソケット部材を取り付けるための、ねじ状部の孔（８）に挿入可能な取付けピン（９）を有する関節ソケット部材（６）とを備える。補綴部材（２）のねじ状部（４）は、補綴部材を大菱形骨（Ｔ）の硬質外側部（Ｔ１）にねじ込み及び締付けによって取り付けるための第１の外側ねじ山（１７）を備えた第１の固定部（１５）と、第１の固定部の径よりも小さい径を有し、補綴部材を大菱形骨の軟質内側部（Ｔ２）にねじ込み及び締付けによって取付けができるように構成される第２の外側ねじ山（１９）を備えた第２の固定部（１８）とによって構成される。第１の固定部（１５）は、固定部（１５、１８）のねじ山（１７、１９）を互いに連結し第２の固定部（１８）に向かって実質的に円弧状に収束する面（２０）を備える。これによって、大菱形骨（Ｔ）における補綴部材（２）の最適な固定が達成される。【選択図】図５
